One sin and your done? No! One sin and its affect separates us from God? - Yes! The question is: 'can continual sin (our continual NO to God) by our volitional right given us by God, take us from our guaranteed position of Salvation, by choosing to not be in Christ?'
we must not confuse position with experience. When a person believes in the Lord Jesus, although he is still very weak and ignorant, God has placed him in a position of being reckoned as having completely died, resurrected, and ascended with the Lord. Those who are accepted in Christ are accepted just as Christ is; this is position.
However, the believers may not necessarily have the experience of this. Positionally speaking, a believer in the Lord possesses all the experiences of the Lord Jesus. Experientially, at the minimum, he is born again. This rebirth is not because he has already experienced the Lord Jesus' death, resurrection, and ascension to a certain degree, but because he believes in the Lord Jesus. His position causes him to have the experience of being born again. Although in experience he still does not know the power of Christ's resurrection (Phil. 3:10), he has already been made alive, raised, and seated in the heavenlies together with Christ (Eph. 2:5-6).